DISTRIBUTION OF MOLLUSCA. 499 
The different genera of the Mollusca are peculiar to, or most fre- 
quent in, certain localities, and even species and varieties are known 
to have their peculiar limits. This fact pervades the entire range of 
organic beings, from the lowest plants to man. The geographical dis- 
tribution of the Mollusca is perhaps that best known to science. The 
labours of Messrs. Louis Agassiz and R. M‘Andrew, Dr. J. E. Gray, 
Professor Edward Forbes, and others, have done much towards 
giving us a clear idea of their distribution in space. Climate alone is 
insufficient to account for the distribution of animals: some higher 
cause rules here. But while we admit this, still we must acknow- 
ledge that climate exerts considerable influence in modifying species. 
The distribution of the Mollusca may be considered from three 
points of view. First, as regards space; second, as regards depth; 
and third, as regards ¢zme, the last belongs to geology. 
We shall now survey the principal divisions of the ocean; the 
line of demarcation being drawn, not by latitude or longitude, but 
by genera and species. 
The Mollusca of the arctic seas are well known to show con- 
siderable analogy with those of the later Tertiary periods of Europe. 
Hence the great interest connected with their comparison, as it affords 
—provided we are satisfied with this line of argument—a proof that 
an arctic climate formerly existed in temperate regions. It is the 
northern Drift of which we are speaking. Even when species are 
found living in Britain identical with those of the arctic regions, still 
there is often a difference in the form or size of British and arctic 
specimens; certain species, such as Cyprina Islandica, being com- 
paratively small in the south of Britain, larger in Shetland, and 
attaining their greatest size in Iceland. 
The countries included in the arctic molluscan province are 
Lapland, Iceland, Greenland, the west coast of Davis’ Straits, and 
Behring’s Straits. About 200 species are enumerated by the various 
arctic voyagers, as found in these seas; of these about one-half are 
peculiar to them, and the other half are either found living in the 
temperate regions of Europe, or in their so-called glacial strata. 
The Boreal province includes the North Atlantic, from Nova 
Scotia to Iceland, and from thence to Faroe, Shetland, and the 
Norway coast. 
The number of species is very large ; and more than one-half are 
common both to Scandinavia and the North American coast, while a 
great number also are found on the British coast. 
The province called Celtic by Professor Edward Forbes embraces 
the coasts of Britain, Sweden, and Denmark. 
GG 2 
